2.4 Risk Assessment
A risk assessment is formally an estimation of the types and the
degrees of danger posed by a hazard. It comprises three elements, which are:
1) hazard identification, 2) risk and vulnerability estimation, and 3)
evaluation of the social consequences (Ferrier and Haque 2003). The general
formula that arises from this conceptual approach is R = pxV, where R
represents risk, p the probability of occurrence, and V, vulnerability to loss.
This formula is substantiated and enhanced in the manual for policy makers and
planners of the United Nations Agency regarding disaster mitigation, which in
addition to hazard and vulnerability inserts element at risk (UNDRO 1991, Diley
et al. 2005, BMRC 2006):
Risk = Hazard x Element at Risk x Vulnerability
Methodologies applied in risk determination include the
stochastic and the systematic approach (Ferrier and Haque 2003, UNDP 2004,
Dilley et al. 2005). The stochastic (or quantitative) method involves
estimating the probability of occurrence and intensity of a hazard, based on
historical data. One of the major weaknesses of this approach is the
insufficient length of historical information, and even its non-existence in
some areas (Huppert and Sparks 2006). In addition, for non-frequent events,
some physical processes such as deforestation, urban sprawl, extension of
impervious areas, and construction in high slopes may affect vulnerability of
places to a certain hazard. Thus, projection of zones susceptible to
disasters based solely on past occurrences may be misleading. The systematic
or deterministic method depends on prior knowledge about the physical
conditions and processes that control the chance of the occurrence of a hazard
(El Morjani 2007). This type of information may be more accessible. An
integrated approach was used by Diley et al. (2005) in a study sponsored by the
World Bank. The application of either or both methodologies relies heavily on
the information situation at hand. Given the strict limitation of historical
and detailed spatial data regarding environmental health hazards we are dealing
with for the study area, this paper relies mainly on the deterministic
approach, which offers the advantage of being integrative, and does not
necessitate factual and historical information.
2.5 Hazard identification or delineation
Hazards can be characterized by their event frequency and
associated characteristics, their probability of exceeding a certain threshold,
and their probability of occurrence based on a range of physical factors (UNDP
1994, Dilley et al. 2005). These concepts were previously generalized in
Hansen (1984) and Hansen and Frank (1991) which classified hazards
determination into two approaches: indirect/causal and direct/occurrence. The
earlier is based on a priori knowledge of the underlining factors of hazards in
the area under study and involves two sub-approaches: heuristic and
statistical. In the heuristic approach factors are ranked and weighted based
on their assumed importance in causing the hazards; in the statistical approach
the role of each factor is determined in comparison to the observed relations
with past/present distribution of the hazard. The fundamental principle of the
direct/occurrence approach relies on the observed distribution of the hazards
over time.
Whereas the advantages of these techniques are unquestionable,
some drawbacks are data availability particularly for small areas, validation
of the information, time consumption for data collection, and precision. Error
in mapping can influence the predictive ability of the model that may not be
possible to be extrapolated to other areas. But the most important limitations
are spatial scale and availability of reliable historic data (Dilley et al.
2005). The intricate context of data collection at spatial and temporal scale
in health hazards may make it even more inappropriate.
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